Radiation therapy is often used to treat patients having malformations such as benign and malignant tumors. In a typical radiation treatment program, a radiation beam of sufficient strength is repeatedly focused on the targeted tumor for a selected period of time. Over the course of several treatment sessions, the focused radiation beam kills tumor cells, ultimately eliminating the tumor. One problem with radiation treatment is that the radiation is harmful to the healthy tissue surrounding the tumor. The radiation must pass through the healthy tissue surrounding the tumor in order to reach the tumor, and over the course of time, the healthy tissue suffers adverse effects and may eventually be damaged or destroyed as a result of repeated exposure to the radiation beam. The tissues surrounding brain tumors, such as the optic nerve, auditory nerves, and the brain stem are particularly susceptible to damage by radiation.
One solution to the above problem has been to irradiate the tumor from a range of angles by moving the patient and the source of the radiation relative to each other. As the radiation beam and the patient move relative to each other, the tumor remains within the beam's width, while at each angle a different portion of the surrounding tissue is irradiated by the beam. In this manner, the radiation required to treat the tumor passes through a greater number of regions of surrounding tissue, which reduces the amount of radiation absorbed by any one region and minimizes the likelihood that the healthy surrounding tissue will be damaged by the radiation.
One problem with the above method of radiation treatment is that it is preferably fractionated to include a series of radiation doses spread out over the course of several days, weeks or months. During each radiation treatment session, the patient and particularly the tumor must be precisely aligned relative to the radiation source in order to ensure that the tumor and only a minimum amount of surrounding tissue are irradiated by the radiation beam. Typically, frame-type devices have been used to fix the position of the patient's head relative to the radiation beam source. These devices are screwed directly into the patient's skull. The target is then located with respect to the frame device. The frame device is cumbersome, uncomfortable, and was not intended to be left on the patient for more than one treatment session, making fractionated treatment impossible.
To solve the positioning problem described above, the assignee employs a frameless stereotactic system. This system uses three metallic markers which are permanently placed in the patient's skull and which define a three-dimensional coordinate system. An imaging device precisely locates the tumor relative to the markers prior to the first therapy session. Once the position of the tumor relative to the markers is known, the radiation beam may be positioned relative to the permanently placed markers rather than the frame device.
In a typical installation of the system described above, the radiation beam passes from the radiation source through a collimator which focuses or directs the beam. The focused beam is then directed to the tumor. The collimator includes a removable die with an aperture therethrough which is used to shape and size the radiation beam, particularly for irregularly shaped tumors. For example, the die may have an aperture that has a size and shape which matches a silhouette of the irregularly shaped tumor when the tumor is viewed from a particular angle. When the tumor is irradiated by the radiation beam at that angle, only the tumor and surrounding tissue directly in the path of the beam are exposed to the beam. When the beam source and collimator are moved so as to irradiate the tumor from a different angle, the die is removed from the collimator and a new die having an aperture corresponding to a silhouette of the tumor when viewed from the new angle is inserted.